This investigation will examine the factors which control the developmental fate of cells in the embryo. The neural crest is chosen as the model system for study because it gives rise to diverse derivatives--including neurons, pigment cells, and cartilage--and is accessible to experimental manipulations. Clones of neural crest cells, derived from a single precursor, will be implanted into host embryos by a recently developed injection technique; in this way, it is possible to explore aspects of differentiation in cells of homogeneous origin. Quail crest cells will be used as donor cells since they can be distinguished from those of the chick host by a heterochromatin marker unique to quail cells. The proposed experiments will examine: 1) if some neural crest cells can give rise to multiple phenotypes at the time of departure from the neural tube; 2) if some crest derivatives are predetermined prior to migration; and 3) the effects of different environments on phenotypic expression in neural crest clones. By using cloned cells which are allowed to differentiate in the natural embryonic environment, this proposal aims to explore and elucidate the factors controlling cell differentiation.